The invention relates to rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutters mounted on the bit body, each cutter having a cutting face, and means for supplying drilling fluid to the surface of the bit body to cool and clean the cutters.
The invention is particularly, but not exclusively, applicable to drill bits in which some or all of the cutters are preform (PDC) cutters each formed, at least in part, from polycrystalline diamond. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the from cutting face of the element, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix.
While such PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations and soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters may suffer accelerated wear and bit life can be too short to be commercially acceptable.
Studies have suggested that the rapid wear of PDC bits in harder formations is due to chipping of the cutters as a result of impact loads caused by vibration, and that the most harmful vibrations can be attributed to a phenomenon called "bit whirl". Bit whirl arises when the instantaneous axis of rotation of the bit precesses around the central axis of the hole when the diameter of the hole becomes slightly larger than the diameter of the bit. Bit whirl may be initiated, for example, when the drill bit meets a harder occlusion or stringer in the formation which obtrudes into the borehole, at least initially, in only one area of the bottom or sides of the borehole. As each cutter strikes the occlusion or harder formation the bit will try to rotate about the cutter which is for the time being restrained by the harder formation, thus initiating bit whirl.
When a bit begins to whirl some cutters can be moving sideways or backwards relative to the formation and may be moving at much greater velocity than if the bit were rotating truly. Once bit whirl has been initiated, it is difficult to stop since the forces resulting from the bit whirl, such as centrifugal forces, tend to reinforce the effect.
One method which has been employed to overcome the bit whirl is to design the drill bit so that it has, when rotating, an inherent lateral imbalance force which is relatively constant in direction and magnitude. The gauge structure of the bit body then includes one or more low friction bearing pads which are so located as to transmit this lateral imbalance force to the part of the formation which the bearing pad is for the time being engaging. The low friction bearing pad thus tends to slide over the surface of the formation which it engages, thereby reducing the tendency for bit whirl to be initiated.
However, this concept relies on a combination of the weight-on-bit and cutter layout to create the required out of balance force. The arrangement cannot therefore become operative to inhibit bit whirl until sufficient weight-on-bit is established. Furthermore, the necessary out of balance force results in excessive friction between the gauge and the walls of the borehole.
In an alternative approach, bits have been designed in a manner to provide a structure which constrains the bit to rotate truly, i.e. with the axis of rotation of the bit coincident with the central axis of the borehole. One such approach is described in Patent Specification No. WO 93/13290.
In PDC bits the cutters are normally arranged in spiral arrays with respect to the central axis of rotation of the bit so that the path swept by each cutter during each rotation overlaps the paths swept by other cutters disposed at slightly greater and slightly smaller radial distances from the bit axis. This provides an essentially smooth cutting profile to ensure that no part of the formation at the bottom of the borehole remains uncut. By contrast the above-mentioned specification proposes a cutter formation where the cutters, instead of being located in spiral formations, are disposed in concentric radially spaced arrays centred on the axis of rotation of the bit. In such an arrangement the cutters in each circular array sweep through essentially the same cutter path and the cutter paths of adjacent arrays do not overlap but are spaced apart in the radial direction. Consequently, the cutters define a series of concentric annular grooves in the cutting profile. As a result the cutters in each circular array cut a deep groove in the formation at the bottom of the borehole with annular ridges of uncut formation extending upwardly between the adjacent circular arrays of cutters.
The presence of the annular ridges increases significantly the vertical contact between the cutters and the formation so that any lateral force acting on the bit, whether externally generated or from cutting structure imbalance, is distributed over a larger contact area. This reduces the unit stress on the formation and the result of lower unit stress is said to result in less tendency for a cutter to bite laterally into the formation and initiate bit whirl.
However, this arrangement limits the depth of cut which can be achieved by individual cutters. This is known to be inefficient and studies have shown that deep cuts are more efficient and that cutter wear can actually increase at small depths of cut.
The present invention sets out to provide a new and improved form of drill bit in which the tendency for bit whirl to be initiated may be reduced, without the problems referred to with respect to the prior art bit stabilising arrangements.